This application claims priority based on German Patent Application No. 10059867.6, filed 30 Nov. 2000.
The invention relates to a controllable amplifier arrangement. The invention also relates to an arrangement for processing electric signals by means of at least one such controllable amplifier arrangement.
In arrangements and systems for receiving high-frequency electric or electromagnetic signals, control circuits with controllable amplifier arrangements are often required for different purposes and at different positions. For example, in such an arrangement or such a system, the amplitude of a received signal is to be maintained constant or to be limited via a control loop for further signal processing, for example, in a demodulator. For larger control ranges of such control loops, controllable amplifiers are required which must be controllable via a large range of values of their gain factor. Such controllable amplifiers may to this end comprise a plurality of controllable amplifier arrangements, also referred to as single amplifier stages, because the desired maximal overall gain of the controllable amplifier cannot often be achieved with a single amplifier stage.
When using such a controllable amplifier in a control loop of the type described, the gain factors of the last single amplifier stages, arranged in a chain, of the controllable amplifier are first reduced when the amplitude of the signal to be processed increases. It has proved that the behavior of the controllable amplifier as regards noise and distortions is then most favorable. Also when the amplitude of the signals to be processed is strongly increased at the input of the controllable amplifier, the gain factor of the single amplifier stages must not fall below a predetermined value because the signal to be processed must be passed through the single amplifier stages with the appropriate amplitude in order that it is still available at the output of the controllable amplifier.
It is an object of the invention to provide a simple and reliable controllable amplifier arrangement as well as an arrangement for processing electric signals by means of at least one such amplifier arrangement, with which a predetermined minimal gain, i.e. a predetermined minimal value of the gain factor can be adjusted.
According to the invention, this object is solved by a controllable amplifier arrangement comprising
a first differential amplifier stage having a first and a second output branch,
a second differential amplifier stage which is coupled to the first output branch of the first differential amplifier stage and has at least a first and a second output branch, at which a first current in the first output branch of the first differential amplifier stage is controllably divided into partial currents,
a third differential amplifier stage which is coupled to the second output branch of the first differential amplifier stage and has at least a first and a second output branch, at which a second current in the second output branch of the first differential amplifier stage is controllably divided into partial currents,
a first load impedance coupled to one of the first output branches of the second differential amplifier stage for generating a first output voltage from the partial current flowing in said one of the first output branches of the second differential amplifier stage, and
a second load impedance coupled to one of the first output branches of the third differential amplifier stage for generating a second output voltage from the partial current flowing in said one of the first output branches of the third differential amplifier stage. In this controllable amplifier arrangement, the first and the second load impedance are bridged to a predetermined part by at least one of the second output branches of the second and third differential amplifier stages, respectively.
In the controllable amplifier arrangement according to the invention, the first differential amplifier stage is used for amplifying an electric signal to be processed, for example in a system for receiving high-frequency electric signals. The second and third differential amplifier stage are used for dividing the output currents of the first differential amplifier stage optionally between the load impedances or for at least partly bridging them. Voltages then occur at the load impedances, which are only generated by the part of the current for the signal to be processed, which current flows through these load impedances. By adjusting this part of the currents, the gain factor of the controllable amplifier arrangement can be adjusted to predetermined values. This adjustment may be performed very precisely because it is based on the adjustment of a current ratio which can be realized very accurately, particularly in the integrated circuit technique.
Moreover, the controllable amplifier arrangement according to the invention uses only a single differential amplifier for amplifying the signal to be processed and therefore has a very simple structure.
According to the invention, the object is further solved by an arrangement for processing electric signals which comprises at least one of the controllable amplifier arrangements described above.
In an advantageous embodiment of the controllable amplifier arrangement according to the invention, said one of the second output branches of the second differential amplifier stage has two jointly controlled sub-branches, a first sub-branch of which is coupled to the first load impedance. Moreover, said one of the second output branches of the third differential amplifier stage has two jointly controlled sub-branches, a first sub-branch of which is coupled to the second load impedance.
In this embodiment of the controllable amplifier arrangement according to the invention, the desired minimal gain, i.e. the desired minimal value of the gain factor is determined by a current ratio in the sub-branches of the second output branches of the second and the third differential amplifier stage. When these sub-branches are particularly provided with bipolar transistors, said current ratio can be predetermined in a very easy and precise manner via the ratios of the emitter areas of these transistors in the different sub-branches.
In a further embodiment of the controllable amplifier arrangement according to the invention, said one of the second output branches of the second differential amplifier stage is coupled to a tap on the first load impedance, and said one of the second output branches of the third differential amplifier stage is coupled to a tap on the second load impedance.
Due to such an arrangement, which also uses only a first differential amplifier stage for amplifying the signal, a further simplification of the circuit is achieved. The minimal gain is predetermined by an impedance ratio which is determined by the arrangement of taps. When particularly ohmic load impedances are used, the minimal gain is determined from a resistance ratio which is reproducible in a very precise way, particularly in the integrated circuit technique. This provides the possibility of maintaining strict tolerances in spite of the lower costs of bulk manufacture.
In an advantageous further embodiment of the arrangement according to the invention, using at least two controllable amplifier arrangements of the type described hereinbefore, the second and the third differential amplifier stage of the controllable amplifier arrangements are controllable by means of common control signals and have control characteristics which are mutually offset in such a way that the partial currents flowing in the output branches of the second and the third differential amplifier stage are reversed in the individual controllable amplifier arrangements at different values of the common control signals.
A desired sequence of the gain control of the individual controllable amplifier arrangements, i.e. single amplifier stages is thereby possible without any additional circuit- technical measures in such an arrangement.
In such an arrangement, the output branches of the second and the third differential amplifier stage are formed with transistors whose main current paths have current-conveying cross-sectional areas whose dimensioning determines the reversal of the partial currents in the output branches of the second and third differential amplifier stages of the individual controllable amplifier arrangements at the different values of the common control signals in the individual controllable amplifier arrangements.
Due to this implementation of the arrangement according to the invention, the described reversal of said partial currents in the individual controllable amplifier arrangements is possible in a very simple and precise manner at different values of the common control signals.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.